The invention relates to an x-ray diagnostic apparatus for producing transverse layer images of radiography subject, comprising an x-ray measuring arrangement containing an x-ray source which produces an x-ray beam penetrating the radiographic subject, the cross-sectional extent of this x-ray beam, perpendicular to the layer plane, being equal to the layer thickness, as well as containing a radiation receiver which determines the radiation intensity behind the subject, comprising means for changing the direction of the symmetry axis of the x-ray beam, and comprising a computer for transforming the signals supplied by the radiation receiver into a tomographic image, wherein the x-ray source is constructed in the form of a ring (or annulus) with an annular anode arrangement, wherein there is disposed in operative association with the anode arrangement a number of cathodes which number is dependent upon the desired measured value number, wherein, in order to change the direction of the symmetry axis of the x-ray beam, means are provided for the stepwise switching on of the electron radiation between at least one cathode in each instance and the anode arrangement, and wherein the radiation receiver, which is likewise ring-shaped, and into the opening of which a support with the radiographic subject fits, is coaxially encompassed by the x-ray source, and consists of a row of individual detectors.
The U.S. Pat. No. 3,778,614 illustrate an x-ray diagnostic apparatus for producing transverse layer images of a radiographic subject comprising an x-ray measuring arrangement containing an x-ray source which produces an x-ray beam penetrating the radiographic subject, and whose cross-sectional extent, perpendicular to the layer plane, is equal to the layer thickness, as well as containing a radiation receiver which determines the radiation intensity behind the subject, wherein the radiation source and the radiation receiver are commonly laterally adjustable and rotatably mounted. During the scanning of the radiographic subject, lateral adjustment movements and rotational movements through a predetermined angle, for example 1.degree. , follow one another in alternate succession until a scan cycle has been completed. From the measured absorption values, a computer computes the transverse layer image in the form of a matrix of image-point-information (or data). In the case of this x-ray diagnostic apparatus, the time required for the radiographic exposure is comparatively long on account of the necessary mechanical movement of the radiation receiver and the x-ray tube. Moreover, a comparatively large constructive outlay is required for mounting the measuring arrangement.
In the publication "Computerized Tomographic Scanner" of the American Science and Engineering, Inc., Publication No. ASE-3869, an x-ray diagnostic apparatus is described wherein a radiation receiver having a ring construction is provided which manifests an opening for receiving a support table with a patient, and which consists of a row of individual detectors. Within this radiation receiver, an x-ray tube is rotatably mounted about the axis of the radiation receiver. In order to examine a radiographic subject, the x-ray tube is rotated through an angle of, for example, 360.degree. about the patient. The radiation receiver is stationary. The measured values supplied by the radiation receiver are again processed by a computer which computes the transverse layer image. The time required for a radiographic exposure and the constructional outlay is decreased as compared with the apparatus known from the U.S. Pat. No. 3,778,614, due to the radiation receiver which is stationary during a scanning cycle.
In order to further reduce the radiographic exposure time, in accordance with the initially described construction, it is also possible to construct the x-ray source in the form of a ring (or annulus) with a ring-shaped anode arrangement, wherein there is disposed opposite the anode arrangement a number of cathodes which is dependent upon the desired measured value number. If the electron radiation is switched on in a stepwise manner between at least one cathode in each instance and the anode arrangement, the direction of the symmetry axis of the x-ray beam can thereby be changed without any mechanical movement of the x-ray source. A further shortening of the radiographic exposure time is thereby rendered possible. However, the problem which arises is that the x-radiation cannot initially pass through the radiation receiver directly subsequent to issuance from the x-ray source, but that it can impinge upon the corresponding number of detectors of the radiation receiver only when it has issued from the radiographic subject.